For example, compared to conventional materials for cutting tools such as ultra-hard tools, etc., cBN based sintered body cutting tools have material characteristics of high performance that can be highly efficient and long-lasting because of the chemical stability and the extreme hardness of cBN sintered body. In addition, cBN sintered body cutting tools are highly valued for their superior flexibility and high environmental-friendly productivity compared to grinding tools, and they have substituted for conventional tools in the machining of hard-to-cut ferrous materials.
cBN sintered body materials can be classified into two types: one type is a sintered body comprising cBN particles and binder materials, in which the cBN content ratio is high, the cBN particles bond each other, and the main components of the remainder are Co and Al as described in Patent Document 1, or is a sintered body that does not comprise any component other than cBN as much as possible, as described in Patent Document 2 (called “high cBN content ratio sintered body” hereinafter). The other type has a comparatively low cBN content ratio, has a low contact ratio between cBN particles, and is bonded through a ceramic comprising Ti nitrides (TiN) and carbides (TiC) that show a low affinity with iron, as disclosed in Patent Document 3 (called “low cBN content ratio sintered body” hereinafter).
In uses in which chips are split off and are not likely to generate shear heat, the former type, high cBN content ratio sintered body achieves outstanding stability and long lifetime because of the superior mechanical characteristics (extreme hardness, high strength, high toughness) and high thermal conductivity of the cBN; and it is suitable for cutting of ferrous sintered parts and gray cast iron in which mechanical wear and damage caused by rubbing against hardened particles and damage caused by thermal impact based on high speed interrupted cutting predominate.
Nonetheless, in machining of steel and hardened steel in which the cutting edge is exposed to a high temperature by large quantity of shear heat produced by continuous cutting, the lifespan is shorter than that of conventional carbide tools and ceramic tools because wear is rapidly advanced by the thermal wear of the cBN component with the iron.
Meanwhile, the latter, low cBN content ratio sintered body manifests superior wear-resistance characteristics based on the workings of ceramic binder comprising TiN and TiC, which have a low affinity with iron at high temperatures, and in particular, in hardened steel machining which cannot be machined practically with conventional carbide tools and ceramic tools, low cBN content ratio sintered body has positively substituted in grinding as a cutting tool that can achieve a tool life ten to several dozen times that of conventional tools.
In recent years, by increasing rigidity of machine tools, adjusting the percentages of cBN and the ceramic binder comprising TiN and TiN in low cBN content ratio sintered bodies, cBN sintered body tools are applied instead of grinding tools to machining applications in which the required precision is 3.2 μm to 6.3 μm in ten point averaged roughness (abbreviated “Rz” hereinafter), for example, as in cutting of automotive transmission parts comprising hardened steel, which is a steel with a surface hardness enhanced to Hv 4.5 GPa to 7.6 GPa by the so-called hardening treatment such as carburized hardening.
Recently, in sliding surfaces and rotating surface, etc. that require a high precision surface roughness of Rz 0.4 μm to 3.2 μm, studies have begun on the application of cutting tools comprising low cBN content ratio sintered body instead of grinding, which has restrictions in terms of machining efficiency and flexibility, in uses for the final finishing step requiring high surface integrity having sufficient fatigue strength in the machined region, or for semi-finish machining to obtain a high surface integrity using only finish processing with an ultra-fine machining allowance of 5 to 10 μm or less, such as machine honing, which needs a smaller machining allowance than the conventional grinding process.    Patent Document 1: Japanese Patent Publication No. S52-43486    Patent Document 2: Japanese Patent Publication No. H10-158065    Patent Document 3: Japanese Patent Publication No. S53-77811    Patent Document 4: Japanese Patent Publication No. H08-119774